Speech and channel encoding schemes in cellular TDMA systems of today use static allocation of source and channel encoding bit rate as well as static allocation of source and channel encoding algorithm. That is, regardless of the changing character of the speech signal and the changing radio conditions an allocated source/channel encoding mode will be maintained for the duration of a call.
However, for certain input source signals the allocated source encoding bit rate may not be sufficient, or the source encoding algorithm may not be adequate, to encode the signal with the desired high quality. Furthermore, when radio conditions become poor the received speech quality will also bee poor, since the protection provided by the channel encoding will be insufficient. Thus, the static mode allocation is a limiting factor with respect to the range of conditions over which the speech transmission service can offer acceptable quality.
Thus, there is a need for a more flexible source/channel encoding mode control system that dynamically adapts, within the same total gross bit rate (the total bit rate available for information transfer on a given channel if no channel protection is provided), the encoding mode to prevailing conditions, such that more channel protection is used on poor channels than on good or almost error free channels, where the bits instead may be used to increase the quality of the source coding.
Citation [1] describes a speech/channel encoding mode control system in which different encoding modes are used for voiced and unvoiced speech, such that voiced speech uses less bits for speech encoding and more bits for channel encoding than unvoiced speech. During time periods of poor radio conditions the encoder is instructed to override this approach by using the more protected mode regardless of the voiced or unvoiced nature of the speech signal.